Frac plug mill bit

ABSTRACT

A fixed cutter bit for milling a frac plug includes a body and a face. The face includes a base surface and a plurality of cutter support structures extending from the base surface. Each cutter support structure has a peripheral portion and an inner portion disposed radially internal of the peripheral portion. At least one first-type cutter is supported by each peripheral portion; at least one second-type cutter is supported by each inner portion. The first type cutter is adapted to mill a harder material than the second-type cutter, and the first-type is different from the second-type.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to U.S. patent application Ser. No.13/975,094 filed on Aug. 23, 2013, and entitled Hybrid Rotary Cone DrillBit, the disclosure of which is hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates generally to bits for drilling a wellbore,and more particularly to a frac plug mill bit for use in drilling outhydraulic fracturing equipment (e.g. frac plugs) or bridge plugs.

BACKGROUND

Different types of earth boring drill bits are used for drilling throughdifferent materials in in oil, gas, and mining fields operations tobreak through earth formations to shape a wellbore. In shaping thewellbore, the bit drills through different geological materials makingup different rock formations. Although the drill bit encountersdifferent formations at different depths in drilling through rock,generally speaking all parts of the drill bit are drilling the same typeof rock formation at the same time.

In hydraulic fracturing operations, a frac plug is secured to a casingthat lines the borehole. The frac plug is something of a disposable toolbecause after the frac plug has performed its function, it is drilledout, and the drilled out pieces of the plug are flushed up the wellboreby the drilling mud. A frac plug is a generally cylindrical componentformed of different materials disposed at different radial positionsmoving from a generally hollow center. For example, a cast iron or otherhard material generally annular slip is disposed at a radial perimeterof the frac plug. In contrast to drilling through rock formations, whendrilling out a frac plug, the drill bit simultaneously drills throughsignificantly different materials. The different materials createdifferent penetration efficiencies and wear characteristics on differentparts of the bit.

Conventional roller cone rock bits manufactured to InternationalAssociation of Drilling Contractors (IADC) standards are used to drillout frac plugs. However, conventional roller cone rock bits were notdesigned for the bi-modal material encountered in drilling out fracplugs. Moreover, roller cone rock bits include relatively small journalbearings and a fragile lubrication system that may be have a higherincidence of malfunction or failure when subjected to the high forces ofdrilling through a cast iron slip portion of a frac plug.

Relatively long tapered milling tools are also sometimes used to drillout frac plugs. Such milling tool may be set with crushed tungstencarbide, tungsten carbide chisels, discs, or PDC cutters. In operation,long, tapered milling tools may over torque and over-strain a smalldiameter positive displacement motor and small diameter tubingassociated with the long, tapered mill tool.

Reference is made to U.S. Pat. No. 4,538,691 to Dennis, the disclosureof which is incorporated by reference, which discloses a drill bit fordrilling earth formations. The drill bit includes cutter elements thatare disposed in a peripheral edge, a plurality of inclined side walls,and in a floor of a recess portion of the bit. The cutter elements areconventional polycrystalline diamond studs. Dennis discloses that theconfiguration of the drill bit, particularly a protrusion at its center,creates a generally balanced pattern of forces that tend not to divertthe drill bit from its intended path of travel.

Reference is made to U.S. Pat. No. 7,958,940 to Jameson, which disclosesan apparatus to drill through a composite frac plug. The apparatusincludes a plurality of blade-type cutter support structures. One of thecutter support structures extends beyond the center of the mill toprovide cutting action at the center. The support structures aredisposed at between 18 and 26 degree angle with the horizontal in orderto cut plug material from the outside to the inside of the frac plug.The support structures support conically shaped carbide inserts orcutters. Each blade-type cutter support structure may support only onecutter at a radial perimeter of the bit. Thus, according to the teachingof Jameson, five support structures collectively support five cuttersradially disposed to attack the slip of a frac plug.

SUMMARY

In an embodiment, a fixed cutter bit for milling a frac plug includes abody and a face. The face includes a base surface and a plurality ofcutter support structures extending from the base surface. Each cuttersupport structure has a peripheral portion and an inner portion disposedradially internal of the peripheral portion. At least one first-typecutter is supported by each peripheral portion; at least one second-typecutter is supported by each inner portion. The first type cutter isadapted to mill a harder material than the second-type cutter, and thefirst-type is different from the second-type.

The frac plug mill bit of the present disclosure is employed to drillout different materials of a frac plug simultaneously. The location ofthe first-type of cutters, referred to as hard material cutters, on aperipheral portion of the face of the bit, and the radial location ofthe second-type of cutters, referred to as soft material cutters,disposed on the inner portion of the face of the bit facilitate drillingout different materials of the plug. Specifically, the relatively hardermaterial of a plug slip disposed on an outer diameter of the plug iseffectively drilled out by the hard material cutters disposed on theperipheral portion of the bit, while the relatively softer material ofthe plug body is effectively drilled out by soft material cuttersdisposed radially inward of the hard material cutters on the innerportion of the face of the bit.

The term “hard material cutters” is used herein to refer to cutters thatare particularly suited to fracture and break apart the hard material ofthe slip, as opposed to the comparatively softer material of the plugbody. The term “soft material cutters” is used herein to refer tocutters that are particularly suited to tear and shear the comparativelysofter material of the plug body. The terms “hard material cutters” and“soft material cutters” are not intended to describe the materialcomposition of the particular cutters.

According to one embodiment, the concave profile created by wedge-shapedcutting structures allows the hard material of the slip of the frac plugto be drilled out ahead of softer material of the plug body. That is,the hard material cutters lead the softer material cutters as the millbit is advanced within the wellbore. Also, the peripheral surface ofeach wedge cutting structure supports multiple hard material cutters. Inthis manner, the number of hard material cutters employed to attack theslip of a frac plug may be significantly increased.

Other aspects, features, and advantages will become apparent from thefollowing detailed description when taken in conjunction with theaccompanying drawings, which are a part of this disclosure and whichillustrate, by way of example, principles of the inventions disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following briefdescription, taken in connection with the accompanying drawings anddetailed description, wherein like reference numerals represent likeparts, in which:

FIG. 1 illustrates a frac plug mill bit disposed in a drill out positiondirectly above a cross section of a frac plug set in a borehole;

FIG. 2 illustrates an isometric view of the frac plug mill bit of FIG.1;

FIG. 3 illustrates a face of the frac plug mill bit of FIGS. 1 and 2;

FIG. 4 illustrates a cross section showing the concave profile of thefrac plug mill bit of FIGS. 1-3;

FIG. 5 illustrates an isometric view of an alternate embodiment of afrac plug mill bit; and

FIG. 6 illustrates a cross section of the frac plug mill bit of FIG. 5.

DETAILED DESCRIPTION OF THE DRAWINGS

Reference is now made to FIG. 1, which shows a frac plug mill bit 10 ina borehole or wellbore 12 lined with a metal casing 16. The bit 10 isshown in a drill out position above a cross section of a casing plug,frac plug, or plug 14.

The frac plug mill bit 10 is a fixed cutter-type bit. The bit may be arandom cutter bit where cutters are individually set on the bit, or thebit may be a blade-type bit where defined blades support the cutters.The bit 10 includes a bit body 18 formed from a matrix metal or anyother material suitable for earth boring drill bits. The bit body 18includes a plurality of wedge-shaped cutting structures defining aperipheral surface 20. The peripheral surface 20 supports a plurality ofcutters 22 disposed closest to the casing 16 in the wellbore 12, whichare adapted to cut or mill hard material. The cutting structuresdisposed more toward the center of the bit 12 are adapted to mill softermaterial.

The frac plug mill bit 10 is configured to drill out the entirety of aborehole and/or a frac plug 14 secured within a borehole. Thus, the fracplug mill bit 10 is configured to drill out either rock formation orportions of the frac plug from the centerline of the borehole andextending to the full radius of the borehole.

In certain borehole operations, such as hydraulic fracturing orfracking, a plug 14, such as a frac plug, is used to isolate a portionof a wellbore 12 to be fracked. The plug 14 acts as a one-way valve andallows a specific section of the borehole to be isolated and pressurizedfor the hydraulic fracking operation. After the plug 14 has performedits function, it is drilled out in a drill out operation using the fracplug mill bit 10 according to the teachings of the present disclosure.In a drill out operation, the frac plug mill bit 10 is attached to adrill string and is rotated such that its cutting elements crush, rip,and break apart the plug 14. Drilling fluid pumped through the bit 10flushes the pieces of the plug 14 back to the surface. Plugs other thanfrac plugs may be secured in a borehole and may be drilled out with thefrac plug mill bit 10 according to the teachings of the presentdisclosure. For example, the frac plug mill bit 10 may be used to drillout bridge plugs and other types of plugs that engage a casing 16.

In preparation for fracking, the plug 14 is positioned at the desiredlocation in the borehole 12 such that an outer diameter portion of theplug 14 grips the casing 16 and secures or sets the plug 14 in position.Once set, the plug 14 will withstand pressurization of the zone in theborehole without moving or slipping. To set the plug 14, a slip 26 thatis generally in the form of a ring surrounding a portion of a plug body28 is caused to engage the casing 16 and create a type of seal. Forpurposes of this disclosure, the plug body 28 includes any portion ofthe plug not formed of relatively harder material that is engaged withthe casing 16 to set the plug in position and create a seal. Althoughthe plug body 28 is primarily disposed radially internal to the slip 26,some portions of the plug body 28 may be disposed above or below andaligned with the slips 26.

In the embodiment illustrated in FIG. 1, an upper and a lower slip 26are shown. The slips 26 each include a plurality of ridges 29 that biteinto the casing 16 to provide a robust grip. The slips 26 expand and maypartially fracture such that some of the slips 26 embed into the metalcasing 16. To maintain the grip of the plug 14 under high pressures, theslip 26 is generally formed from a hard material. In certain plugs 14,the slip 16 is formed from cast iron. Once set, the slip 26 occupies aspace between the casing 16 and the plug body 28, which may be up to aninch inside the diameter of the casing. For example, a casing 16 of aborehole may have a diameter of approximately twelve inches and the slip26 may have an outer diameter of approximately twelve inches and aninner diameter of approximately ten inches.

In certain embodiments, the slip 26 may include tungsten carbide orceramic inserts that embed into the casing 16 for a better grip. A plugincluding such inserts is disclosed in U.S. Pat. No. 5,984,007 to Yuan(the disclosure of which is incorporated by reference). In contrast tothe very hard material of the slip 26, the plug body 28 is generallyformed of softer material than the slip 26 and/or any inserts that areincluded in the slip 26. For example, the plug body 26 is often formedof a composite material, a thermoplastic, phenolic, or a softer metal,such as brass. Because the plug 14 includes relatively softer materialsin its inner portions and relatively harder materials in its outerportions, during drill out, the frac plug mill bit 10 simultaneouslycontacts and breaks apart both relatively harder and relatively softermaterials.

Reference is made to FIGS. 2 and 3. FIG. 2 is an isometric view of thefrac plug mill bit 10, and FIG. 3 illustrates a face 30 of the frac plugmill bit of FIG. 1. The frac plug mill bit 10 includes a bit body 18 anda face 30. A plurality of cutter support structures 32 are generallywedge-shaped and extend from a generally convex base surface 34.Together the cutting wedges 32 form a generally concave cuttingstructure. The bit 10 may include between two and six cutter supportstructures or cutting wedges 32. In the embodiment illustrated, the face30 of the bit 10 includes three cutting wedges 32. Each cutting wedge 32includes a peripheral surface 20 that merges into an inclined surface36. The inclined surface 36 is delimited on one end by the peripheralsurface 20 and on an opposite end by the base surface 34. The transitionbetween the peripheral surface 20 and the inclined surface 36 may berounded or the junction of the two surfaces may intersect at a moredefined edge. The peripheral surface 20 is generally perpendicular to arotational axis of the bit 10, and the inclined surface 36 forms anangle θ with respect to a horizontal (see FIG. 4). According to certainembodiments, θ is at least 30 degrees. For example, θ may be between 30and 45 degrees.

The bit 10 also includes a plurality of ports or nozzles 38. Drillingfluid is pumped down the drill string and flows through the nozzles 38where it can direct cuttings and other debris through junk slots 40 andback up the borehole. In the embodiment illustrated, each wedge 32includes a nozzle 38, and an additional nozzle 38 is disposed near thecenter of the bit 10. However, more or less nozzles 38 may be employedaccording to the teaching of the present disclosure. A junk slot 40 isdisposed between adjacent cutting wedges 36 and a floor of the junk slotis a portion of the base surface 34. The number of junk slots 40generally corresponds to the number of cutting wedges 32. Thus, threejunk slots 40 are illustrated. However, the bit 10 may include betweenone and six junk slots 40, depending on the number of cutting wedges 32.In the illustrated embodiment, the three junk slots 40 are each largeenough to allow reasonably large portions of the broken-up slip 26 topass through.

Another portion of the base surface 34 is a dome-shaped protrusion 42disposed at the center of the face 30. This protrusion 42 functions tobring high point loading to frac plug balls 15 (see FIG. 1) encounteredwhen drilling each frac plug. The center protrusion 42 also assists indriving a remainder of a lower portion of a drilled plug to engage anext plug deeper in the wellbore. To ensure full coverage of cutterssupported by the bit 10, the protrusion 42 supports a plurality ofcutters. In the embodiment illustrated, the protrusion 42 supports aplurality of cylindrical cutters that have a smaller diameter than thehard material cutters 22 supported by the peripheral surface 20.

The peripheral surface portions 20 of the cutting wedges 32 support aplurality of cutters 22 disposed in respective cutter pockets 23 togenerally form a ring around the perimeter of the bit 10. The cutters 22may be disposed in their respective cutter pockets 23 at any suitableback rake angle. The cutters 22 supported by the peripheral surface 20are adapted to cut or mill hard material, such as the cast iron slips 26imbedded or otherwise contacting the casing 16. These hard materialcutters 22 are disposed on the wedge such that they will cut the fracplug material nearest the casing 16. Each cutting wedge 32 may supportmultiple hard material cutters 22 on its portion of the peripheralsurface 20. In the embodiment illustrated, each cutting wedge 32supports three hard material cutters 22 for a total of nine hardmaterial cutters 22 disposed on the peripheral surface 20 and radiallypositioned to attack the slip 26. The increased number of cuttersdisposed to attack the slip 26 increases the effectiveness and theuseful life of the frac plug mill bit 10 when used to drill out fracplugs 14. Larger diameter bits 10 may support more hard material cutters22.

Reference is now made to FIG. 4, which is a cross section of the bit 10.The hard material cutters 22 are supported by a peripheral portion 35 ofthe cutter support structure 32, which in the illustrated embodimentgenerally corresponds to the peripheral surface 20. The peripheralportion 35 extends radially internal from the gage 33 and supportscutters 22 that are disposed to cut within approximately 1.5 to 2 inchesof the casing 16. Alternatively, the radial distance of the peripheralportion 35 may be 1 to 0.5 inches, and therefore the hard materialcutters 22 supported by the peripheral portion 35 are disposed to cutwithin 1 to 0.5 inches of the casing 16. As described in more detailbelow, an inner portion 37 is radially internal to the peripheralportion 35 and includes soft material cutters 24 adapted to tear andshear the softer material of the plug body 28.

The hard material cutters 22 may be cylindrical cutters. For example,the hard material cutters 22 may be any one of the followingcutter-types: cubic boron nitride, tungsten carbide, tungsten carbideprotected polycrystalline diamond compact (PDC), or coated tungstencarbide. According to one embodiment, a diamond layer of the PDC cutterincludes a milling cap such as a tungsten carbide cap, a tungstencarbide or cubic boron nitride tipped cap, or a similar fitted cap madeof a suitable material and fitted as an integral part of an existing PDCcutting structure, as described in U.S. Pat. No. 8,517,123 to Reese,which is hereby incorporated by reference.

In the illustrated embodiment, the inner portion 37 includes theinclined surface portions 36 of the cutting wedges 32 and the basesurface 34. The inner portion 37 supports a plurality of cutters 24 alsodisposed in respective cuter pockets 23. More specifically, in oneembodiment, the cutters 24 supported by inner portions 37 are adapted tocut through and mill the softer material that forms the inner portionsof the body 28 of the frac plug 14. For example, these soft materialcutters 24 may include standard PDC cutters, chisel or scribe shaped PDCcutters, chisel shaped tungsten carbide cutters, and disc shapedtungsten carbide cutters. According to an embodiment, the soft materialcutters 24 are scribe cutters and may or may not be shark tooth-typecutters. The scribe cutters are pointed cutters that are particularlyuseful in deforming the softer material of the frac plug, such asplastic, such that the softer material shears and tears apart and can beflushed up the wellbore.

In an alternate embodiment, the soft material cutters 24 and the hardmaterial cutters 22 may both be PDC cutters. However, the soft materialcutters 24 are still a different type of cutter because they have ascribe shape that is particularly suited to shear and tear the softermaterial of the plug body 28. Similarly, the soft material cutters 24and the hard material cutters 22 may both be PDC cutters, yet the hardmaterial cutters may have a higher diamond composition than the softmaterial cutters 24 that makes the hard material cutters more suitablefor fracturing and breaking apart the cast iron or other hard materialof the slips 26 with reduced abrasion or other wear on the cutters.

The bit 10 may also include studs, such as Teflon studs, disposed on itsouter periphery. The studs are not disposed to cut through the frac plug14, but rather are disposed to maintain centering of the bit 10 in thecasing 16. In addition, the bit 10 may include hemispherical tungstencarbide inserts disposed in the leading edge of the junk slots 40, or onthe outer periphery of the bit 10 to assist in smoothly centering thebit 10 inside the casing 16 of the wellbore.

The generally concave shape of the cutting wedges 32 (see FIG. 4), andtherefore the cutting structures of the bit 10 allow the hard materialcutters 22 supported by the peripheral surface 20 to drill the slips 26ahead of the drilling performed by the soft material cutters 24. Assuch, the more robust and stronger slips are fractured and removed aheadof the body 28 of the plug. When the last of the slip material isdrilled in a specific plug the remaining body portion rides down thewellbore with the mill to engage the top of the following plug to bemilled.

Each cutter 22, 24 is secured in its respective cutter pocket 23 usingbonding techniques that are known in the art for the particular type ofcutter being bonded. For example, the cutters 22, 24 may be secured intothe cutter pockets 23 by brazing, welding, or adhering using anadhesive. The cutter pockets 23 are formed by casting or machining andare sized to receive a corresponding cutter 22, 24.

Reference is now made to FIGS. 5 and 6. FIG. 5 is an isometric view ofan alternate embodiment of a frac plug mill bit 50, and FIG. 6illustrates a cross section of the frac plug mill bit 50 of FIG. 5. Thefrac plug mill bit 50 is a fixed cutter bit. The frac plug mill bit 50includes a bit body 52 and a face 54. Cutter support structures 56extend from a base surface 55 and support a plurality of cutters, someof which are adapted to cut and fracture the hard material of the slip26, and others are adapted to tear and shear the plug body 28, dependingon the radial location on the bit of the particular cutter. According tothis embodiment, the cutter support structures 56 are referred to asblades.

FIG. 6 is a cross section showing two blades. In the illustratedembodiment, the gage 58 portion of the blade cutter support structure 56supports a pre-flatted gage cutter 60 and a full diameter gage cutter62. Radially internal to the gage 58 is a shoulder 64. The shoulder 64is generally an arcuate surface disposed between the gage 58 and thenose 66. In the embodiment illustrated, the shoulder 64 supports a neargage cutter 68. Radially internal to the shoulder is the nose 66 and thecone 70. The illustrated embodiment is a flat profile bit, but theteachings of the present disclosure apply to concave profile bits andfixed cutter bits having other profiles.

In order to fracture the hard material of the slip 26 when drilling outthe frac plug 14, a peripheral portion 72 of each cutter supportstructure 56 supports a first-type of cutter 74 that is particularlysuited to fracture and split apart the hard material of the slip 26. Theperipheral portion 72 includes the gage 58 and the portion of the cuttersupport structure 56 extending radially inward approximately 1.5 to 2inches from the gage 58. In this manner, the hard material cutters 74are disposed to cut and mill a slip with a radial thickness ofapproximately 1.5 to 2 inches. The peripheral portion 72 also may extendradially inward from the gage 1 inch or 0.5 inches, according toalternate embodiments. In the embodiment illustrated, the cutterssupported by the gage 58 and the shoulder 64 are hard material cutters74 and the cutters supported radially internal to the shoulder at aninner portion 76 are soft material cutters 78 adapted to tear and shearthe softer material of the plug body 28. In the illustrated embodiment,the inner portion 76 includes the nose 66 and the cone 70 portions.

Similarly as stated above, the hard material cutters 74 may be any oneof the following cutter-types: cubic boron nitride, tungsten carbide,tungsten carbide protected polycrystalline diamond compact (PDC), orcoated tungsten carbide. According to one embodiment, a diamond layer ofthe PDC cutter includes a milling cap such as a tungsten carbide cap, atungsten carbide or cubic boron nitride tipped cap, or a similar fittedcap made of a suitable material and fitted as an integral part of anexisting PDC cutting structure, as described in U.S. Pat. No. 8,517,123to Reese.

The soft material cutters 78 include standard PDC cutters, chisel orscribe shaped PDC cutters, chisel shaped tungsten carbide cutters, anddisc shaped tungsten carbide cutters. According to an embodiment, thesoft material cutters 78 may or may not be shark tooth-type cutters.

The frac plug mill bits 10, 50 each differ from a reamer in that areamer is not configured to drill out a central portion of a boreholeproximate the centerline. Rather, a reamer is configured to ream a holethat has already been at least partially formed. However, with respectto the cutting dynamics and force balancing of the bit, the frac plugmill bits 10, 50 are similar to a reamer in that, when drilling out afrac plug, they are subjected to greater forces near their respectiveperimeters and reduced forces nearer their centers.

To that end, the mill bits 10, 50 of this disclosure may be forcebalanced by modifying the teachings of U.S. Pat. No. 8,162,081 toBallard, in connection with the teachings of U.S. Pat. No. 5,010,789 toBrett, the disclosures of each of which are hereby incorporated byreference. The high compressive strength of the outer ring of the fracplug is modeled, along with the low compressive strength of the innerplug material to produce a force balancing of the overall cuttingstructure which overcomes the tendency to have “bit whirl” induced whiledrilling the bi-modal materials of the frac plug. For example, the forcebalance model may include parameters associated with a 1.5 inchthickness slip, such as 15,000 pounds/sq. inch as a value for thecompressive strength of the slip. This thickness corresponds to aperipheral portion 35, 72 defined as the region 1.5 inches radiallyinternal to the gage. The inner portion 37, 76 may be force balanced fora plug body having a compressive strength of 5,000 pounds/sq. inch. Thesame modeling procedure may be used to model a peripheral portion 35, 72having a distance reduced by 0.5 inches to one inch, and the innerportion distance can be increased by 0.5 inches. This model correspondsto a slip having a radial thickness of one inch. This iterative modelingtechnique may be repeated for any suitable peripheral portion and usingany suitable incremental distance. In this manner, the bit 10, 50 may beforce balanced and adapted to attack frac plugs having slips of avariety of different radial thicknesses.

According to one embodiment, force balancing leads to a bit design thatis densely set with cutting structure in the outer ring andsignificantly more lightly set with cutting structure across theremainder of the bit face.

According to embodiments of the present disclosure, during drill out ofthe frac plug 14 using the frac plug mill bit 10, 50 hard materialcutters 22, 74 supported by peripheral portions 35, 72 of cutter supportstructures 32, 56 engage the slip 26 and/or the plug inserts that areadjacent, contacting, or embedded into the casing 16. Softer materialcutters 24, 78 are supported by the inner portion 37, 76 and are adaptedto drill out softer materials of frac plug body 28.

The foregoing describes only some embodiments of the invention(s), andalterations, modifications, additions and/or changes can be made theretowithout departing from the scope and spirit of the disclosedembodiments, the embodiments being illustrative and not restrictive.

What is claimed is:
 1. A fixed cutter bit, comprising: a body; and aface, comprising: a base surface; a plurality of cutter supportstructures extending from the base surface, each cutter supportstructure having a peripheral portion and an inner portion disposedradially internal of the peripheral portion; at least one first-typecutter supported by each peripheral portion; and at least onesecond-type cutter supported by each inner portion, the first-typecutter adapted to mill a harder material than the second-type cutter,the first-type being different from the second-type.
 2. The bit of claim1 wherein the plurality of cutter support structures form a generallyconcave cutting portion of the face.
 3. The bit of claim 2 wherein eachcutter support structure includes an inclined surface having an angle ofat least 30 degrees from a horizontal.
 4. The bit of claim 1 wherein thebase surface is generally convex and the peripheral portions includeperipheral surfaces that are generally perpendicular to a rotationalaxis of the bit.
 5. The bit of claim 1 wherein the first-type isselected from a group consisting of: polycrystalline diamond cutters,cubic boron nitride cutters, cutting tool grade carbide cutters, andmill capped polycrystalline diamond cutters.
 6. The bit of claim 5wherein the second-type is either tungsten carbide cutters orpolycrystalline diamond cutters.
 7. The bit of claim 1 wherein theperipheral portions extend between 0.5 and 1.5 inches from a gagesurface of the bit.
 8. The bit of claim 1 wherein the cutter supportstructures are blades.
 9. The bit of claim 1 wherein the first-typecutters supported by the peripheral portions and the second-type cutterssupported by the inner portions are set such that the bit is forcebalanced when milling a slip having a thickness between 0.5 and 1.5inches.
 10. The bit of claim 9 wherein the cutters supported by theperipheral portions are more densely set than the cutters supported bythe inner portions.
 11. The bit of claim 1 wherein the peripheralportions collectively support at least six cutters.
 12. A fixed cutterbit for milling a frac plug, comprising: a body; and a face having aperipheral portion and an inner portion disposed radially internal tothe peripheral portion, the peripheral portion supporting a plurality offirst-type cutters and the inner portion supporting a plurality ofsecond-type cutters, the first-type being different from thesecond-type; the second-type cutters adapted to mill a plug bodymaterial; and the first-type cutters adapted to mill a slip material,the slip material having a greater compressive strength than the plugbody material.
 13. The bit of claim 12 wherein the first-type cuttersand the second-type cutters are individually set.
 14. The bit of claim12 wherein the first-type cutters and the second-type cutters aresupported by blade-type cutter support structures.
 15. The bit of claim14 wherein the blade-type cutter support structures each include a gageportion and an arcuate shoulder portion, the gage portions and theshoulder portions supporting the first-type cutters.
 16. The bit ofclaim 12 wherein the bit has a flat profile.
 17. The bit of claim 12wherein the bit has a concave profile.
 18. The bit of claim 12 whereinthe first-type is selected from a group consisting of: polycrystallinediamond cutters, cubic boron nitride cutters, cutting tool grade carbidecutters, and mill capped polycrystalline diamond cutters.
 19. The bit ofclaim 12 wherein the second-type is either tungsten carbide cutters orpolycrystalline diamond cutters.
 20. The bit of claim 19 wherein thesecond-type are scribe-shaped cutters.
 21. A fixed cutter bit formilling a frac plug, comprising: a body; and a face having a peripheralportion and an inner portion disposed radially internal to theperipheral portion, the peripheral portion supporting a plurality offirst-type cutters and the inner portion supporting a plurality ofsecond-type cutters, the first-type being different from thesecond-type; the second-type cutters adapted to mill a plug bodymaterial; the first-type cutters adapted to mill a slip material, theslip material having a greater compressive strength than the plug bodymaterial; wherein the first-type is selected from a group consisting of:polycrystalline diamond cutters, cubic boron nitride cutters, cuttingtool grade carbide cutters, and mill capped polycrystalline diamondcutters; wherein the second-type is either tungsten carbide cutters orpolycrystalline diamond cutters; and wherein the first-type cutters andthe second-type cutters are set such that the bit is force balanced whenmilling a slip having a thickness between 0.5 and 1.5 inches.